In electrical equipment such as air conditioners, freezers, and heaters, micro switches have found advantageous application in temperature control including thermostat control, which enables on-off control of large amounts of electrical power without using a relay. Such micro switches have a fixed contact, a movable contact, and a snap-action device. The snap-action device moves the movable contact in or out of contact with the fixed contact to open and close an associated electric circuit.
The fixed and movable contacts of the micro switch have been conventionally made of silver, a silver-carbon sintered metal, a silver-metal oxide alloy, or the like. Particularly, most micro switches have been made of a silver-metal oxide alloy that exhibits outstanding performance with respect to mutual welding of the contacts and consumption of the contacts due to arc discharge.
Japanese Patent Application Laid-open No. 59-123108 discloses a pair of contacts each of which is made of a material having a different characteristic. One of the contacts is made of a silver-tungsten carbide (Ag-WC) sintered metal, while the other is made of an Ag-SnO.sub.2 alloy (viz., a silver-metal oxide compound alloy.
A commonly used micro switch for a large current has been made of a silver-metal oxide alloy that has an outstanding performance against mutual welding of the contacts and against consumption by arc discharge. However, the alloy has the disadvantage of a larger contact resistance to cause a substantial temperature rise at its associated contacts when energized as compared with silver or a silver-carbon sintered metal.
Thus, when both the movable contact and the fixed contact are made of silver-metal oxide alloy, a contact point temperature of the pair of contacts becomes elevated because of a larger contact resistance, disadvantageously affecting an electrical performance thereof. To solve this problem, it is required that the pair of contacts have a larger diameter to increase the contact area thereof for reducing the contact resistance, or that there is provided with an auxiliary plate having a high thermal conductivity for heat radiation.
The enlargement in diameter of the contacts without modifying the switch case in size, decreases clearances between the contacts and other parts, causing a possibility of an arc discharge therebetween. Particularly, a micro switch having a snap-action device consisting of a resilient plate and a U-shaped, snap-action spring raises the problem that the snap-action spring is located very close to the fixed contact, inviting an arc discharge therebetween. Since the snap-action spring is joined to a free end portion of the resilient plate supported by the switch case at one end thereof, and is joined to a movable contact supporting plate having a contact at the other end thereof, the fixed contact must be partially cut away in a side thereof opposed to the snap-action spring, for example, into a D-shape configuration.
Meanwhile, contacts which are both made of a silver-carbon sintered metal do not encounter the above problems. However, the silver-carbon sintered metal, is disadvantageous as compared with a silver-metal oxide alloy in connection with mutual welding of the contacts and consumption of the contacts due to arc discharge. The contacts made of a silver-carbon sintered metal thus have a shorter life. In addition, a silver-carbon sintered metal has a comparatively higher toughness, which hinders plastic molding and is not suitable for a die-cut flat conductor or a contact plate because they can not be directly crimped onto a contact supporting plate.
Japanese Patent Application Laid-open No. 59-123108 discloses a pair of contacts, one of which is made of a silver-tungsten carbide sintered metal, while the other is made of a silver-metal oxide compound alloys. However, this combination is not suitable with respect to preventing mutual welding of the contacts and contact resistance to obviate heating of the contacts. Moreover, it requires a tungsten carbide that has been heat-treated at a high temperature.